This invention relates to a process for producing an aluminum alloy bearing which comprises bonding an aluminum alloy layer to a backing metal layer composed of steel and further coating the surface of the aluminum alloy layer with a dam layer and an overlay In particular, it relates to a de-smutting treatment to be carried out in the middle of the course of the above production process, by which treatment, the smut is removed which remains on the surface of the aluminum alloy layer after the surface of the aluminum alloy layer has been etched for enhancing the adhesive force of the intermediate layer of a half-finished product in which the aluminum alloy layer is bonded to the backing metal layer.
As an aluminum alloy bearing 1, there has been broadly known one obtained by bonding an aluminum alloy layer 12 to a backing metal layer 11 and further applying an intermediate layer 13 and an overlay 14 to the internal surface of the aluminum alloy layer 12 by plating as shown in FIG. 1 of the accompanying drawings.
These aluminum alloy bearings 1 are explained below referring to a semi-cylindrical half bearing 1a as shown in FIG. 2 of the accompanying drawings. Two half bearings 1a are assembled as one set in the cylindrical internal surface of a housing 20 and support a rotating shaft 21 so that the shaft can be freely rotated. In this case, such difficulties are caused that when the peripheral length of the bearing is longer than the length of the internal periphery of the housing 20, the bearing is greatly deformed to the inside, and when it is contrarily shorter, the position of the bearing is shifted during the sliding, whereby it becomes impossible that the bearing smoothly supports the rotating shaft 21. Therefore, the dimensional tolerance of the external diameter of the half bearing 1a to the internal diameter of the housing 20 is required very critically.
For example, as to a bearing having an internal diameter of 40 to 80 mm, JIS D3102 defines the tolerance of the height H of the bearing as within 0.04 mm. The height H of this bearing, as shown in FIG. 3, becomes the external surface radius of the half bearing 1a when this half bearing is contained in the concave of the semi-circular arc shape of a cylindrical measuring jig 31 having the given diameter and pressed at the given pressure P by means of a press plate from one side.
In the process for producing such an aluminum alloy bearing, the aluminum alloy layer 12 is bonded to the backing metal layer 11 and the resulting laminate is cut to a ribbon shape, and this ribbon-shaped plate is subjected to bend forming to a half cylinder shape, thereby obtaining a semi-finished product 10 as shown in FIG. 4 of the accompanying drawings. Thereafter, the internal surface of the semi-finished product 10 is coated with an intermediate layer 13 and an overlay 14 in this order as shown in FIG. 1 by a plating method.
In the above production process, for the purpose of enhancing the adhesive force between the aluminum alloy layer 12 and the intermediate layer 13, an etching treatment is conducted for dissolving and removing oxides and the like formed on the surface of the aluminum alloy layer 12 as a preliminary step before forming the intermediate layer 13 on the aluminum alloy layer 10 by plating. The etching treatment is effected by immersing the semi-finished product in an alkali solution. However, insolubles (smut) such as copper, silicon and the like which are the added elements in the aluminum alloy layer 12 are attached to and remain on the surface of the aluminum alloy layer, and a de-smutting treatment is subsequently conducted for removing the insolubles remaining on the surface.
The de-smutting treatment is conducted by contacting the semi-finished product with a de-smutting solution. As the de-smutting solution, an aqueous solution comprising a combination of an acid and an oxidizing agent has heretofore been used, and a de-smutting solution composed of a combination of sulfuric acid and bichromic acid, a combination of nitric acid and hydrofluoric acid, or the like has been generally used.
Since in the de-smutting treatment, not only the aluminum alloy layer but also the backing metal has been contacted with the de-smutting solution, when a de-smutting solution composed of a combination of sulfuric acid and bichromic acid, a combination of nitric acid and hydrofluoric acid or the like has been used, even the backing metal has been corroded simultaneously with the removal of the smut. When the backing metal 11 has been corroded to be made thin, the bearing height H has been decreased and the variation of dimension has become large.
Moreover, JP-A 7(1995)-233,490 discloses the use of a mixed aqueous solution of phosphoric acid and nitric acid as the de-smutting solution and states that the corrosion of the backing metal by the de-smutting treatment can be inhibited. Furthermore, it states that by adding a nitrogen-containing organic compound such as urea or the like, the generation of nitrous acid gas is inhibited, whereby the deterioration of the working environment can be prevented.
Recently, even in the production of a bearing, various measures have been taken from the viewpoint of preventing the deterioration of working environment and the generation of environmental pollution.
The conventional de-smutting solution composed of a combination of sulfuric acid and bichromic acid, a combination of nitric acid and hydrofluoric acid or the like has caused a large corrosion of the backing metal, and, in addition, when bichromic acid is used as an oxidizing agent, it has been necessary to remove chromium (VI) compounds and thereafter drain water. Furthermore, when nitric acid is used, it is necessary to prevent the deterioration of the working environment due to the generation of nitrous acid gas.
It is also stated that the de-smutting solution composed of the mixed solution of phosphoric acid and nitric acid disclosed in JP-A 7(1995)-233,490, when used in combination with a nitrogen-containing organic compound, can prevent the generation of nitrous acid gas. However, since nitrogen and phosphorus are contained, it has such a problem that waste management is required to be sufficiently effected. Moreover, phosphoric acid is generally expensive, and hence, in the above technique in which a large amount of phosphoric acid is used, there is such a fear that the product may become high in cost